The works have included the revamp of the ground and first floor to provide consultation rooms while the top floor now houses open plan office and meeting space for staff.
The developer says the new spaces have been designed to maximise natural light and to create a calm internal environment.
The new-look office - officially known as 17 Bridge Street - will become the base for about 300 staff of Cambridgeshire and Peterborough NHS Foundation Trust’s adult mental health and social care teams.
The public will also use the premises for outpatient appointments.
Steph Stebbings, the trust’s estates manger (North), said: “Bridge Street offers improved facilities for our teams, and also for our service-users because outpatient appointments will take place there.
“To have been able to continue this project throughout the pandemic has been very challenging but it’s great that we are now able to relocate our teams to their new home.”
The trust’s former homes at the Gloucester Centre, in the Ortons, and its Ida Darwin site in Cambridge, have been sold for £20 million and will make way for housing development.
The renovation works began after Peterborough City Council moved most of its staff out of the Town Hall to new offices at Sandmartin House at Fletton Quays in 2019.
Most of the development costs have been met by the council with the remainder paid by the trust.
Representatives of R G Carter joined officials from the council, the trust and architects Hamson Barron Smith to celebrate the completion of the project.
Martin Randall, client manager of R G Carter, said: “This has been an exciting and truly collaborative project for us to be involved in and we are delighted to be able to deliver much-needed facilities for the region.
“With all our projects we aim to use local trade to support local jobs and the economy, and we are thrilled that over 85 per cent of the sub-contract values on the development was sourced within 45 miles.
“It’s always a great feeling to hand over a project that matches the client’s vision for the very best services and support that will make a real impact to the wider community.”
Peter Carpenter, the council’s corporate director of resources, said: “We are delighted with the refurbishment of the Town Hall.
“Since moving most of our operations out of the Town Hall we had been looking at different options for making use of the building.
“Having the CPFT move in will be of great benefit to residents in need of their vital services.”